Patent GB-1066609-A (GB)
Title: Gas turbine engine fuel control
IPC: F02C9/16
Filed: 1965-03-19
Abstract: 1,066,609. Gas turbine engine fuel control systems. HOLLEY CARBURETOR CO. March 19, 1965 [March 30, 1964], No.11725/65. Heading F1G. [Also in Division G3] A fuel control system for a gas turbine engine controls the supply of fuel from a pump 54 to the engine in dependence upon compressor discharge pressure, compressor speed, power turbine speed and ambient temperature. As shown, the pump 54 delivers fuel to the turbine via a spring-loaded valve 110, a chamber 70, a metering valve 172 having a linear pressure drop/flow characteristic, a spring-loaded valve 200, a passageway 402 and a spring-loaded valve 394. The stem 176 of the metering valve 172 abuts a pivoted lever 252 connected to a compressor speed-sensitive diaphragm 250 and a variable spring load via a hand-lever 14, cam 262 and spring 264. The chamber 70 contains a valve stem 72 having closure surfaces 98, 100 co-operating with by-pass ports 90, 92, the stem being connected to a lever 68 pivoted at 74 and to a diaphragm 130. The diaphragm 130 separates two chambers 132, 134, the former containing an evacuated bellows 116. The chambers communicate with compressor discharge pressure via a restriction 138 and passageway 136, the supply to chamber 132 containing a further restriction 142 which provides for a faster initial response of the valve stem 72 to changes in compressor discharge pressure downstream of restriction 138 (C.D.P 1 ). The free end of lever 68 is connected to a diaphragm 150 which is responsive to the pressure drop across valve 172. In operation, variation in C.D.P 1  varies the by-pass flow and, consequently, the pressure P 2  acting on diaphragms 150, 250, the former acting to provide a feedback to lever 68 whilst the latter adjusts the position of the metering valve 172. To increase speed, lever 14 is rotated to cause the cam 262 to move lever 252 away from the stem of the metering valve, thereby allowing the latter to open to increase the supply of fuel. The chamber 244 defined by diaphragm 250 communicates with the delivery side of the pump 54 via a restriction 238 and with chamber 70 via passages 266, 240 and a compressor speed responsive valve 218. Therefore, as the speed increases and valve 218 closes, the increased pressure difference across diaphragm 250 provides a feedback to lever 252 to move the latter back into contact with the stem of the . metering valve to adjust the position of the latter to the new position corresponding to the increased fuel requirement. During the acceleration, the increased pressure difference across diaphragm 150 due to the increased pump delivery pressure would tend to open the by-pass valves, but this is balanced by the increased C. D. P 1  acting in chambers 132, 134. The diaphragm 130 will actuate the by-pass valves in response to variation in C. D. P 1  caused by variations in altitude to maintain the engine speed constant. The maximum fuel flow to the engine is controlled by providing a spring 124 between the flange 128 on the end of the by-pass valve spindle and the diaphragm retainer 122, the spring being arranged to compress above predetermined value of C.D. P 1  to thereby limit the minimum opening at the by-pass ports. Minimum fuel flow is controlled by providing a bleed port 204 in valve 200, the pressure difference across the port being communicated to a spring loaded diaphragm 156. Thus, if the pressure P 3  is too low to open valve 200 and the said pressure difference falls below a predetermined value, a spring 158 will move diaphragm 152 and a projection 162 on the latter will close the by-pass valves. The speed, and hence the power output, of the power turbine is controlled by arranging the latter to drive a governor 270 which actuates a hinged valve 318 to modulate the value of the C.D.P 1  in chambers 132, 134. The spring loading or the governor is controllable through a lever 298 and cam 302 by a hand-lever 16. The value of the C.D.P 1  in chambers 132, 134 may also be modulated by valves 366, 334 controlling flow in a passageway 326 communicating with passageway 136. Valve 366 is actuated by a diaphragm 350 responsive to the pressure difference across the compressor speed responsive valve 218, and valve 334 is actuated by bimetallic discs 342 responsive to ambient temperature. In operation, at compressor speeds below a first predetermined value the leak from passage 326 is controlled by valve 334, the latter functioning, at a given speed, to give a value of the ratio: Fuel Flow/C.D.P 1  which is dependent on the ambient temperature, the value of the ratio being between so-called Hot Day and Cold Day limits Fig. 7 (not shown). At speeds above the first predetermined value but below a second predetermined value, the speed responsive value 366 begins to close, thereby raising the effective value of the C.D.P 1  in chambers 132, 134 and, consequently, the value of the said ratio. As the valve 366 closes, the effect on the ratio of valve 334 reduces until, at speeds above the second predetermined value, the valve 366 is closed completely and valve 334 has no effect. An automatic shut-off valve 374 may be provided which, when opened by cam 262, allows fuel from chamber 70 to pass, via passages 266, 380 and 406, to a chamber 388 housing valve 394. This high pressure fuel, together with spring 400 then urges valve 394 to the closed position. In the alternative embodiment of Fig. 3 (not shown), valves 366 and 334 are replaced by a single valve 458 actuated by a three-dimensional cam 426, the cam being moved linearly by temperature sensitive element 324 and rotated by a speedresponsive piston 424, the latter replacing the diaphragm 350 referred to above. This arrangement provides the more complex relationship between Fuel Flow/C.D.P 1  and compressor speed of Fig. 8 (not shown). To arrange for the ratio Fuel Flow/C.D.P 1  to fall within the curves V-R, V-T of Fig. 8 during starting of the engine, a diaphragm 470 responsive to the pressure difference across the compressor speed-responsive valve 218 is provided, the diaphragm being connected with lost motion to an extension 468 of lever 68. The ratio is thus controlled during start-up by the diaphragm 470, which prevents C.D. P 1  from exerting its full effect on the by-pass valve, and by the temperature responsive element 324. When a speed corresponding to R, T is reached, the diaphragm 470 no longer has any Influence. Also shown in Fig.3 is a solenoid-actuated valve controlled e.g. by the pilot, which controls the supply of fuel from chamber 182 to an igniter via a pressure regulating valve 490.